Solid-state imaging devices with higher resolutions are used in many commercial applications, especially in camera uses and also in other light imaging uses. Such imaging devices typically comprise of CCD (charge coupled device) photo detector arrays with associated switching elements, and address (scan) and read out (data) lines. This CCD technology has matured so much that now millions of pixels and surrounding circuitry can be fabricated using CMOS (complimentary metal oxide semiconductor) technology. Since today's CCD technology is based on silicon (Si)-technology, the detectable spectral ranges of CCD are limited to the wavelengths below 1 μm where Si exhibits absorption. Additionally, CCD based imaging techniques have other shortcomings such as an inability to achieve high efficiency response combined with high quantum efficiency over broad spectral ranges. This broad spectral detection is required in many applications. One of them is in free space laser communication where shorter (in visible ranges) and near infrared wavelengths are expected to be used. Photodiode arrays having broad spectral detection capabilities, disclosed in this invention, are expected to provide those features not available in today's CCD technology. With a well designed array, appreciable resolution can also be achieved in photodiode array technology.
Photodiodes, especially of p-i-n type, have been studied extensively over the last decade for their applications in optical communication. These photodiodes are for near infrared detection, especially the wavelengths in the vicinity of 1310 and 1550 nm, with which today's optical communication is dealt. Nowadays, photodetector speeds as high as 40 Gb/s, as described in the publication by Dutta et. al. in IEEE Journal of Lightwave Technology, vol. 20, pp. 2229-2238 (2002), are achieved. Photodetectors having a quantum efficiency close to 1, as described in the publication by Emsley et. al., in the IEEE J. Selective Topics in Quantum Electronics, vol. 8(4), pp. 948-955 (2002), are also available for optical communications. These photodiodes use InGaAs material as absorption material, and the diodes are fabricated on InP wafers. On the other hand, Si substrate is used for the photodiode to detect visible radiation.
For covering broad spectral ranges, two photodiodes, fabricated from Si and InP technology and discretely integrated, can be used. Monolithically, wafer bonding technology to bond Si and InP can be used to fabricated the photodiode covering the wavelengths from visible to near infrared. However, the reliability of wafer bonding over a wide range of temperatures is still an unsolved issue, and high-speed operation is not feasible with a wafer bonding approach. It is highly desirable to have a monolithic photodiode array, which could offer a high bandwidth (GHz and above) combined with high quantum efficiency over broad spectral ranges (less than 300 nm to 1700 nm and also less than 300 nm to 2500 nm). For use especially in imaging purposes, where CCD based devices are used, the photodiode array with the capability to rapidly and randomly address any pixel is also very much essential.
It is our objective to develop a monolithic photodiode array for broad spectral ranges covering wavelength detection from less than 300 nm to 1700 nm, while having a frequency response as high as 8 GHz and above bandwidth and a high quantum efficiency of over 90% over the entire wavelength region.
It is also our objective to develop a monolithic photodiode array (or single detector) for broad spectral ranges covering wavelength detection from less than 300 nm to 2500 nm, while having a frequency response as high as 8 GHz and above bandwidth and a high quantum efficiency of over 90% over the entire wavelength region.
Our innovative approach utilizes a surface incident type (either top or bottom emitting type) photodiode structure having a single set of absorption layers, which can provide broad spectral response. The absorption layers will be designed to achieve the required quantum efficiency and also the required speed. The photodiode can be used as a single element and also in an array form.
In an array form, especially for the case of a top-incident type photodetector, a metal line to connect each pixel separately to the outside contact pads is utilized for making it possible to rapidly and randomly address any pixel independently. As each metal line usually needs to connect outside pads to inside photodiode pixels, the pitch and element size is limited by the width of the metal line and array number. For example, the element size (i.e. photodiode pixel size) and pitch can be made to 5 μm and 10 μm, respectively, for the array size of 25×25 and metal line of 1 μm. For the bottom incident type of structure, a large number of pixels, even an array of over 1000×1000 photodiode elements can be possible.
According to the current invention, photodiodes having broad spectral ranges (less than 300 nm to 1700 nm and also less than 300 nm to 2500 nm), high quantum efficiency (greater than 90%), and high frequency response can be fabricated using the single wafer. According to this invention, in the case of the photodiode array, each array can be operated independently. The manufacturing thereof is also simpler as compared with the prior art. Some applications include multicolor imaging applications, such as for astronomical observation, communication, etc.